Conventionally, for example, in U.S. Pat. No. 6,336,789, a casing of a steam turbine or a gas turbine has been disclosed. In this casing: flanges are provided at both ends thereof in a structure divided into two parts; a shell upper portion having a largely reinforced center portion is provided; and thickness of a lower portion of the shell at a position close to the flanges is kept thin; and thereby, during operation, deformation is caused in the lower portion only, whereas deformation in other parts thereof is made small.